1. Field of the Invention
The invention relates generally to well drilling and well logging. In particular, the present invention relates to displays based on well logging measurements to facilitate well placement during drilling.
2. Background Art
Wellbores drilled through earth formations to drain fluids (such as petroleum) are frequently drilled along a substantially horizontal trajectory in a petroleum reservoir to increase the drainage area in the reservoir. See, for example, “MWD resistivity tool guides bit horizontally in thin bed,” Oil and Gas Journal Dec. 9, 1991. Because petroleum reservoirs are frequently located in layered earth formations, the position of such substantially horizontal wellbores with respect to the boundaries of the layers in the earth formations often has a material effect on the productivity of such wellbores. Estimation of distances to layer boundaries, therefore, is important for well landing and drain-hole positioning.
Techniques known in the art for estimation of the wellbore position with respect to layer boundaries include those which are indirectly based on well logging measurements in close-by (“offset”) wellbores. These techniques assume that the composition and the geometry of the formation layers proximate to the wellbore of interest are substantially the same as in the offset wellbores.
Another group of prior art techniques is based on the observation of features, referred to as “horns,” which appear in measurements made by electromagnetic-type well logging instruments. When this type of instrument approaches a layer boundary between two beds with a (typically large) contrast in electrical resistivity, a significant distortion of the resistivity signal magnitude (a horn) occurs. Qualitative estimates of the distance between the instrument and the layer boundary may be made by observing the magnitude of the horns.
In addition to these techniques that rely on simple distances to the boundaries for well placement, recent techniques use measurements that include three-dimensional information. For example, recent developments in logging while drilling (LWD) tools with tilted antenna indicate that it is feasible to use the improved directional measurements for accurate well placement. U.S. Pat. No. 6,594,584 issued to Omeragic et al. discloses methods using these measurements in real-time to obtain bed boundary distances by inversion calculation.
Geosteering often requires quick decisions. Therefore, it is very important that the relevant information is presented in an intuitive manner. Relevant information needed for accurate well placement may include azimuthal dependence of the directional measurements, inverted distances to bed boundaries, and an improved earth model. During a geosteering job, the geosteering engineer shall be able to assess easily from various displays the distances between the tool and the nearby bed boundaries, and the trend of the well path, i.e., whether the tool gets closer to or farther away from a bed boundary. This kind of visualization capability will allow geosteering engineers to make accurate decisions about adjusting the well path while drilling.